The invention relates to a device for operating an injection valve which can be activated by means of a piezo-actuator, in particular for a spark ignition engine with internal mixture formation, having charging means and discharging means for the controlled charging and discharging of the piezo-actuator to a respectively desired piezo-voltage.
In internal combustion engines with direct fuel injection, the fuel consumption and the emission of pollutants can be optimized by operating injection valves in a defined way during the injection process wherein the injection beam characteristic, such as for example, the opening angle of the beam, the thickness of the beam, the direction of the beam and in particular also the degree of fuel atomization are set in a defined way over the opening stroke of the injection valves. In particular, piezo-actuators, which permit an electrical signal to be converted virtually without delay into a mechanical movement, are suitable for driving injection valves with an adjustable opening stroke. This movement can be controlled precisely in terms of deflection and time profile by adjusting the voltage, which is applied to the piezo-actuator. In this way, injections which are configured in virtually any desired way can be achieved, for example, injections with an opening stroke which decreases continuously between two different opening settings, as disclosed in DR 196 42 653 C1.
An injection valve which is suitable in particular also for use in internal combustion engines with a common rail injection system and which has a piezo-actuator is described, for example, in DE 195 48 526 A1 and in the periodical article M. Rumphorst, xe2x80x9cEin neues elektronisches Hochdruck-Einspritzsystem fxc3xcr Dieselmotoren (A new electronic high pressure injection system for Diesel engines)xe2x80x9d, MTZ Motortechnische Zeitschrift [Motor technology periodical) 56 (1995) 3, page 142. Whereas the valve opens in the system described in DE 195 48 526 A1, when the piezo-voltage is applied, the application of voltage in the system of the aforementioned periodical leads to the valve closing. Here, the discharging process for the piezo-actuator can take place at different voltage levels in order to obtain different opening settings for the valve in this way. The discharging process preferably takes place with a constant discharge flow.
A device for actuating an injection valve which can be operated by means of a piezo-actuator and which corresponds to the type mentioned at the beginning is disclosed in U.S. Pat. No. 5,130,598. In this publication, in order to connect the piezo-actuator in a controllable fashion to a high voltage source and disconnect it therefrom, the voltage applied to the piezo-actuator is compared with a reference voltage representing the desired piezo-voltage. A comparator output signal is used, together with the output signal of an electronic control device, which takes into account engine operating parameters, to actuate a gate circuit in the power path of the high voltage supply for the piezo-actuator. The discharge means make available a high voltage, which corresponds preferably to the maximum desired piezo-voltage, while the discharge potential of the discharge means is preferably at zero potential.
EP 0 460 660 A2 discloses a further device for actuating a piezo-drive for an injection valve, in which the injection valve is closed by applying a high voltage. By monitoring the timing of the discharging of the piezo-actuator to a reference voltage potential, the piezo-voltage, and thus the valve opening stroke, can be continuously set at a specific range.
It is the object of the present invention to provide a device for actuating an injection valve which can be operated by means of a piezo-actuator and which permits specific types of actuation to be implemented with relatively little expenditure and a high degree of reliability.
In a device for controlling an injection valve which can be operated by a piezo-actuator and which comprises a charging and discharging circuit for the controlled charging of the piezo-actuator to a certain piezo-voltage, the charging and discharging circuit includes a power supply with a multitude of voltage levels which correspond to various valve opening positions and a switching device for the controlled switch-over between the plurality of voltage levels.
In one embodiment of the invention, the voltage supply makes available at least three different voltage levels at corresponding terminals between which it is possible to switch over directly in order to provide for different valve opening settings, with the result that a plurality of required valve settings can be set reliably without additional outlay in control.
In another embodiment, time-controlled charging or discharging voltage levels are applied to the piezo-actuator, wherefor only one voltage source is required and with which any desired voltage valve stroke profile can be realized. The charging voltage level is selected to be higher than the maximum desired piezo-voltage, and/or the discharging voltage level is selected to be lower than the minimum desired piezo-voltage. In this way, rapid charging or discharging to a maximum or minimum desired piezo-voltage level is made possible. The time-controlled switching means may permit the potential difference applied to the piezo-actuator to be inverted. This makes it possible to use a voltage source, which only makes available two different voltages.
Preferably, the charging and discharging means have current control means for respectively setting a constant charging or discharging current, the value of the controlled, constant charging current being selected to be equal to the value of the maximum uncontrolled charging current at the maximum piezo-voltage level. This permits the piezo-element to be charged in a non-damaging way.
In a particularly advantageous embodiment of the invention, the charging and discharging means have monitoring means for monitoring the voltage applied to the piezo-actuator and/or the zero position of the piezo-actuator and time setting means, which define the control times for the time controlled switching means as a function of the monitoring function of the monitoring means. This permits the voltage levels to be adjusted adaptively, for example, as a function of fluctuations in the properties of various injection valves and as a function of valve aging phenomena.
Advantageous embodiments of the invention will be described below in greater detail on the basis of the accompanying drawings: